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Focus Control Circuit For Adjusting The Focus By Moving A Lens - Patent 8131143

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Focus Control Circuit For Adjusting The Focus By Moving A Lens - Patent 8131143 Powered By Docstoc
					
				
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Description: S This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2009-167113, filed on Jul. 15, 2009, and Japanese Patent Application No. 2010-127584, filed on Jun. 3, 2010, the entire contentsof which are incorporated herein by referenceBACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus control circuit for determining the focus position by actively moving a lens. 2. Description of the Related Art Most of camera modules installed in commonly-used digital cameras and mobile phones are provided with auto-focusing functions. A contrast detecting method is often used as the auto-focusing function employed in such a compact-size camera. Inthe contrast detecting method, a lens position in which the contrast of a subject lying in a picked-up image frame is maximized is detected by actively moving the lens and then the lens is moved to the detected position.SUMMARY OF THE INVENTION The contrast detecting method can be achieved with less cost than an active method in which the subject is irradiated with infrared rays or ultrasonic waves and the distance to the subject is measured based on the reflected waves. Nevertheless,the contrast method has a drawback in that it takes some time to complete a search for the lens position in which the contrast of a subject is maximized. It is desired that the processing of focusing the subject be completed within one second after auser has pressed a shutter button halfway. The number of pixels of each cameral module installed in the commonly-used digital cameras and mobile phones is increasing every year. As a result, high-resolution images can be easily taken with such a compact-size camera. When thehigh-resolution images are taken, out-of-focus shots may be easily noticeable and therefore further high-precision auto-focus control is required. A focus control circuit according to one embodiment of the present invention is installed in